In a prior art threshold detector (sometimes referred to as a "zero-crossing" detector), the output of a sensor is processed by circuitry such as that shown in FIG. 1.
In the illustrated arrangement, a sensor 10 (such as a reluctance sensor) develops an output signal A which, as shown by waveform A in FIG. 2, appears as a sinusoidal type signal superimposed on a bias level V. At time t.sub.1, the signal A crosses the level V, thereby generating a "threshold-crossing". In the case where the threshold level V is zero volts, the transition at t.sub.1 is referred to as a "zero-crossing". The purpose of the circuitry shown in FIG. 1 is to develop a binary level output signal (waveform D in FIG. 2) having a single "arm" transition and a single "fire" transition (as opposed to multiple, unwanted transitions), even in the case where a substantial amount of noise is present on the input signal A.
Referring again to FIG. 1, the signal A is coupled to the input of a zero-crossing (or threshold-crossing) detector 12 that generates a binary level output signal B (see waveform B in FIG. 2) having a positive-going transition that occurs at time t.sub.1.
The sensor signal A is also applied to an integrator 14 which applies an integrated version of the signal A to a threshold detector 16. The output of the detector 16 is a binary signal C (see waveform C in FIG. 2). This signal C is applied to the clock (C) input of a flip-flop 18, while the signal B is applied to the reset (R) input of the same flip-flop.
The purpose of the flip-flop is to generate a noise-free output signal D (waveform D in FIG. 2) whose "arm" transition establishes an amplitude level from which one can generate the "fire" transition. Although the stability and accuracy of the "fire" transition is important insofar as it gets counted, or otherwise used, to form a timing reference, the stability of the "arm" transition is also important. If the "arm" transition jitters in the presence of noise, the circuitry which processes the signal D may operate improperly. For purposes of this disclosure, a signal which "jitters" is one which has multiple, noise induced, transitions between a high level and a low level, as opposed to a single transition between the same levels.
To stabilize the "arm" transition, the threshold detector 16 typically includes a single comparator with hysteresis. When the comparator fires, it generates the transition 19 (FIG. 2), and the hysteresis associated with the comparator prevents the comparator from generating additional transitions in response to noise that does not exceed the hysteresis range.
Unfortunately, it is impractical to construct a single comparator which can apply sufficient hysteresis in time to reject high levels of noise. Consequently, conventional threshold crossing detectors, such as shown in FIG. 1, develop output signals (waveform D) whose "arm" transitions tend to jitter when moderate or high levels of noise are present.